This patent application is related to a digital system for providing motor bus transfers, and is generally related to U.S. Pat. Nos. 4,310,771; 4,356,972 relating to methods and hardware for transferring power supplied to motors in a synchronous mode from a main utility source to an auxiliary source and assigned to the same assignee as the present invention. The foregoing cited patents disclose analog systems. This patent application is also closely related to U.S. Pat. No. 5,224,011 that discloses a digital system that is a predecessor of the system disclosed in the present application.
In power plant applications the motor load is transferred from auxiliary source to the main source and vice versa during starting and shut-down of the power plant. It is important to minimize motor power interruption time to prevent dropping of this motor load. Also, the transfer of the motor bus from one source to the other should be carried out in a synchronous manner to prevent motor damage.
Voltage sags or total supply interruptions represent a prevalent and critical problem in providing quality energy to industrial loads. Voltage disturbances or interruptions of electronic control systems and other sensitive installations can lead to complete loss of production in a factory facility. Also, it is important to maintain continuity of electric service to industrial facilities such as chemical and petroleum facilities. An outage of service has enormous detrimental consequences. Accordingly, such industrial facilities are normally connected to a main source of utility power and to an auxiliary source of power. When the main source is interrupted or cut off, the motor loads are transferred from the main source to an auxiliary source to provide energy to the motors and other loads. This application describes a new digital system for automatically transferring the motor load from the main source to the auxiliary source with no interruption or motor damage that is when the main source is interrupted the motors are synchronously connected to an auxiliary source.
It is known that when disconnected from a source of energy, rotating equipment will decelerate at a rate which is a function of the initial rotational inertia and the retarding torque. Considering that a typical motor bus has a combination of motor loads connection thereto, the deceleration of the total system becomes a complex function of the total system inertia, the time constants of the individual motor loads, the trapped flux in each machine, as well as the individual characteristics of each machine. As a result, the total motor bus may be viewed as an equivalent machine decelerating at a composite rate. The frequency of the residual voltage present on the motor bus during the de-energized transfer period will decay at a rate determined by a complex “average” of the spectrum of parameters associated with the motor bus. The system frequency will decay as the stored energy is dissipated by the load.
It is known that the determination of the time when the breaker should be closed involves measuring the phase difference between the motor bus voltage and the auxiliary source voltage; calculating the time derivative of the phase difference to obtain the frequency difference; and, correlating these functions. A prediction of zero phase difference is made; and, knowing the breaker closing time, a phase difference point can be calculated at which point it is desirable to initiate the closing of the breaker.